This invention relates to methods for certifying or validating the existence or occurrence of a recorded document or event, particularly methods which rely upon cryptographic assumptions to establish the basis for such a certification or validation. More specifically, the invention relates to a method for reconfirming an original certificate in order to maintain its validity for a significant period of time beyond the probable compromise of an underlying cryptographic assumption or step in the original certification procedure.
Time-stamping procedures described in U.S. Pat. Nos. 5,136,646 and 5,136,647 are representative of a type of certification for which the present method is adapted. Such schemes for setting a reliable time of creation of a document, or providing indisputable evidence against the alteration of a document, generally digital computer data in alphanumeric, pictorial, video, or audio form, depend upon the assumption that there exist cryptographic functions which, when applied to a digital representation of such a document, defy any manner of manipulation which might permit undetectable alterations or falsifications of the original state of document elements. The functional procedures generally exemplified in those disclosures typically provide this required property, since they generate unique certificate statements which essentially can not be duplicated other than from an identical document representation. This security arises from the fact that the derivation or reconstruction of these functions from the products of their application is computationally infeasible. Ultimate achievement of such derivations must be anticipated, however, since a given function or procedure may be fatally flawed or, as is becoming more probable, advancements in computer technology and algorithmic techniques are likely to make more readily available a level of calculating power which enables such derivation.
With compromise of a step or algorithm in a procedural certification function, the possibility arises of generating duplicate certificates or parts thereof from different digital representations, i.e., creating "collisions", and thereby defeating the previously reliable basis for a certification scheme. Substitution of a newer and presumably less vulnerable function in the certification procedure may prevent for some finite time the compromise of future certificates, but the value of past certificates in establishing original creation dates, for example, is all but lost. The present invention, however, provides a means for bridging the technological gap and extending into the era of a newer function or procedure the validity of the original certification.